Catalyst for ammonia oxidation

ABSTRACT

Lithia doped cobalt oxide ammonia oxidation catalyst having a lithium to cobalt atomic ratio in the range 0.6 to 1.5.

This is a division of application Ser. No. 07/468,943, filed Jan. 23,1990, now U.S. Pat. No. 5,041,276.

This invention relates to ammonia oxidation and in particular to acatalyst therefor.

Catalysts for oxidising ammonia, e.g. for nitric acid manufacture,generally comprise noble metals such as platinum, often employed in theform of a gauze formed from metal wires. It has also been proposed toemploy cobalt oxide for this application. In particular, the cobaltoxide, Co₃ O₄, doped with small amounts of promoters has been proposed.Thus, U.S. Pat. No. 3,850,851 discloses the use of this cobalt oxidedoped with 0.1 to 10 atom % of lithia (lithium oxide, Li₂ O), andwherein such catalysts are shown to be more stable, and less prone tosintering than those which are not so doped.

We have found that when cobalt oxide doped with much larger quantitiesof lithia is used as a catalyst in an ammonia oxidation reaction anunexpected improvement in the selectivity of the catalyst occurs.

Accordingly the present invention provides an ammonia oxidation processwherein the ammonia in a flow of a reactant gas comprising ammonia andfree oxygen, is oxidised by said free oxygen in the presence of acatalyst, thereby forming oxides of nitrogen, said catalyst comprisingcobalt oxide doped with lithia and thereby having a lithium to cobaltatomic ratio, characterised in that said lithium to cobalt atomic ratiois in the range 0.6 to 1.5.

Accordingly, the present invention further provides a catalyst suitablefor use in an ammonia oxidation process, said catalyst comprising cobaltoxide doped with lithia, thereby having a lithium to cobalt atomicratio, characterised in that said lithium to cobalt ratio is in therange 0.6 to 1.5.

Catalysts having a lithium to cobalt atomic ratio in the range 0.6 to1.5 thus show improvements in selectivity over catalysts hitherto known.It is however preferred that the lithium to cobalt atomic ratio is inthe range 0.8 to 1.2 and more particularly in the range 0.9 to 1.1.

The selectivity of a given ammonia oxidation catalyst is herein termed Sand is defined as:

    S=[NO.sub.x ].sub.o /([NO.sub.x ].sub.o +[N.sub.2 ].sub.o -[N.sub.2 ].sub.i)

where

[NO_(x) ]_(o) is the volume of nitrogen oxides, e.g. NO₂, and NO, in theoutlet gas;

[N₂ ]_(o) is the volume of nitrogen in the outlet gas; and

[N₂ ]_(i) is the volume of nitrogen in the feed.

The selectivity is affected by a number of factors, such as thecomposition of the catalyst, the temperature at which the reaction isconducted, the composition of the gas undergoing the reaction, and thelength of time during which the gas is in contact with the catalyst.

Another meaningful parameter against which to judge the performance ofan ammonia oxidation catalyst is the relative selectivity. The relativeselectivity is herein defined as the ratio of the selectivity of a givenammonia oxidation catalyst, as determined at a particular temperature,gas flowrate and gas composition to the selectivity of a conventionalplatinum/rhodium catalyst operated at the same gas flowrate, withsufficient conventional platinum/rhodium catalyst being present so as toachieve a contact time of about 300 ms, and the same gas composition butat a temperature at which the conventional platinum/rhodium catalystexhibits maximum selectivity. A conventional platinum/rhodium catalysttypically comprises an alloy of 10% w/w rhodium in platinum in the formof a wire gauze, and exhibits a selectivity of over 0.99 at 900° C.using a reactant gas comprising a mixture of ammonia in air, with anammonia to oxygen ratio of between 0.56 and 0.59, and a contact time ofabout 300 ms.

Catalysts of the present invention generally have relative selectivitiesof at least 1, i.e. the selectivity of the catalysts of the presentinvention are at least as high as the maximum selectivities achieved byconventional platinum/rhodium catalysts.

Cobalt is known to have a number of oxide forms, e.g. CoO, Co₃ O₄. Ingeneral cobalt oxide can be represented by a formula Co_(x) O_(y),wherein x and y represent the number of cobalt and oxygen atomsrespectively.

The following relationship of the lithium to cobalt atomic ratio, hereintermed R, to the atom percent of lithium, herein termed P and theparameters x, and y, may be derived.

    R=(1+y/x)P/(100-1.5P)

Usually the cobalt oxide used as a catalyst in ammonia oxidation has aformula Co₃ O₄, i.e. x is 3, and y is 4. Thus R is given by

    R=7/3P/(100-1.5P)

The 0.1 to 10 atom percent of lithium as disclosed in U.S. Pat. No.3,850,851, corresponds to values of R of between 0.002 and 0.275, whichare significantly less than the lower limit of 0.6 of the range as usedin the present invention. Indeed, the most preferred materials withinthe scope of the present invention have a lithium to cobalt atomic ratioof between 0.9 to 1.1. Such preferred materials thus have a lithium atompercent of between 24.4 to 27.6% and thereby conform approximately to aspinel of the form LiCoO₂.

The catalyst may be in the form of a powder, in which form it isparticularly suited for use in a fluidised bed reactor.

Alternatively, where a conventional fixed bed reactor i employed, thecatalyst is preferably in the form of shaped pieces, e.g. as formed bypelleting techniques. In a further form of this preferred embodiment thecatalyst is in the form of extrudates, particularly extruded cylindershaving a multiplicity of through passages, for example the shapesdescribed in EP-A-222541. Alternatively the catalyst may be in the formof a porous foam as described in EP-A-260826.

The lithia doped cobalt oxide may be made by the techniques described inthe aforesaid U.S. Pat. No. 3,850,851 or by forming a mixture of finelydivided lithia and cobalt oxide, or compounds that are decomposablethereto by heating, and then heating the mixture to 700°-900° C.

The ammonia oxidation may be carried out in conventional fashion e.g. bypassing a mixture of ammonia and a free oxygen-containing gas, e.g. air,through a bed of the catalyst at an elevated temperature, usually in therange 700°-900° C., and at a pressure in the range from 1 to 20 bar abs.The ammonia to oxygen ratio of the mixture, wherein the free oxygen isexpressed as O₂, is typically in the range 0.1 to 1.5. Where air is usedas the free oxygen-containing gas, the ammonia concentration ispreferably less than about 10% by volume in order to avoid the formationof explosive mixtures. The concentration of 10% v/v of ammonia in airtherefore gives rise to an ammonia to oxygen ratio of approximately0.55.

The invention is illustrated by the following examples.

EXAMPLE 1

A series of lithia doped cobalt oxide catalysts were made by addinglithium carbonate and cobalt oxide (Co₃ O₄) in the required proportionsto triple distilled de-ionised water. The water was then boiled offuntil a slurry was formed. The slurry was then dried at 110° C. for 24hours. The resultant powder was then fired in a platinum crucible at900° C. for 3 hours. The fired product was then ball milled for 2 days.

The resultant catalysts were tested for ammonia oxidation by placingabout 0.7 g of the catalyst in a reactor and passing a reaction gas,comprising 90% v/v air and 10% v/v ammonia at a rate of 1 liter perminute through the catalyst bed at atmospheric pressure and an inlettemperature of 700° C. The outlet gas was analysed using gaschromatography and the selectivity, S, determined.

The ammonia oxidation was also performed at an inlet temperature of 800°C. The results were as shown in the following table.

    ______________________________________                                        Li/Co ratio      Selectivity                                                  (atomic)         700° C.                                                                        800° C.                                       ______________________________________                                        0.33*            0.89    0.71                                                 0.64             0.89    0.73                                                 0.97             0.98    0.86                                                 1.62*            0.61    0.60                                                 ______________________________________                                         *comparative examples                                                    

The catalyst having a lithium to cobalt atomic ratio of 0.97 was thusshown to have the highest selectivity.

EXAMPLE 2

In this example, a range of catalysts was prepared in which the lithiumwas added by impregnation of cobalt oxide with lithium nitrate solution.The impregnated cobalt oxide was then dried at 120° C., and the lithiumnitrate decomposed to the oxide by heating to 750° C. for 12 hours.

The catalyst was then tested in a "pulse" type reactor. A sample ofcatalyst 4 mm in depth was supported on quartz wool in a reactor of 10mm inside diameter. The temperature of the sample was increased to, andmaintained at 800° C. An inert gas stream (helium) was passed though thesample at a rate of 2.18 liters per minute per square centimeter of flowarea. A pulse of 5 ml of a reaction gas consisting of 10% v/v ammonia,20% v/v oxygen and 70% v/v helium was then injected into the inert gasstream flowing through the sample. The change in the nitrogen oxidecontent of the gas exit the sample was monitored by mass spectroscopy.

A conventional platinum/rhodium catalyst was used in a similar "pulse"type reactor, but was maintained at the temperature at which maximumselectivity was exhibited for the conventional catalysts i.e. about 900°C. The change in nitrogen oxide content of the gas exit the sample wasagain monitored by mass spectroscopy.

The relative selectivity of the lithia doped catalyst was derived fromthe ratio of the area of the trace for the change in nitrogen oxidecontent obtained when using the lithia doped catalyst to that obtainedwhen using the conventional catalyst.

    ______________________________________                                                      Relative                                                        Li/Co ratio   Selectivity                                                     (atomic)      @ 800° C.                                                ______________________________________                                        0.00*         0.84                                                            0.06*         0.89                                                            0.11*         0.90                                                            0.31*         0.94                                                            0.66          1.01                                                            1.07          1.06                                                            1.48          1.00                                                            ______________________________________                                         *comparative examples                                                    

It is thus evident that catalysts according to the present invention arecapable of greater selectivity than conventional platinum/rhodiumcatalysts.

EXAMPLE 3

In this example further samples of the catalyst as formed in Example 1,and having a lithium to cobalt atomic ratio of 0.97, were tested forammonia oxidation at differing ammonia to oxygen ratios.

The conditions for the testing of the samples were similar to those ofExample 1, except that a 2% v/v oxygen in nitrogen mixture was passedthrough the sample of catalyst at a flowrate of 1 liters per minute, andthe appropriate amount of ammonia was introduced into the gas mixture.

The results are shown in the following table.

    ______________________________________                                        Ammonia to       Selectivity                                                  Oxygen ratio     700° C.                                                                        800° C.                                       ______________________________________                                        0.5              0.97    1.00                                                 0.75             0.95    0.97                                                 1.0              0.95    0.95                                                 1.5              0.84    0.80                                                 ______________________________________                                    

It is seen that this material is very highly selective over a wide rangeof ammonia to oxygen ratios.

EXAMPLE 4

In this example the catalyst was prepared using equimolar proportions oflithium carbonate and cobalt oxalate, which were ball milled for 24hours in ethanol and then the resultant mixture was fired in air at 900°C. for 11 hours. The resulting material has a composition approximatingto a spinel, LiCoO₂, as shown by XRD.

This material was tested using a gas mixture of 10% v/v oxygen inhelium, with the addition of ammonia at the level required to achievethe various ammonia to oxygen ratios. The total flowrate of gas to thesample was maintained at 0.1 liters per minute, so as to achieve acontact time with the catalyst sample of 425 mS. the results are shownin the following table:

    ______________________________________                                        Ammonia to       Selectivity                                                  Oxygen ratio     700° C.                                                                        800° C.                                       ______________________________________                                        0.5              0.72    0.95                                                 0.75             0.61    0.96                                                 1.0                      0.58                                                 1.5                      0.50                                                 ______________________________________                                    

We claim:
 1. A catalyst capable of catalysing the oxidation of ammoniaby free oxygen, said catalyst comprising cobalt oxide doped with lithia,thereby having a lithium to cobalt atomic ratio, and wherein saidlithium to cobalt ratio is in the range 0.6 to 1.5.
 2. A catalyst asclaimed in claim 1 wherein said lithium to cobalt ratio is in the range0.9 to 1.1.
 3. A catalyst as claimed in claim 1, which, when used tocatalyse the oxidition of the ammonia by the free oxygen in a flow of areactant gas consisting of 10% v/v ammonia, 20% v/v free oxygen, and 70%v/v helium at a temperature of 800° C., exhibits a selectivity at leastas high as that achieved by a wire gauze when sufficient of said wiregauze, to obtain a contact time of 300 ms with said flow of saidreactant gas, is used to catalyse the oxidation of the ammonia in saidreactant gas at a temperature at which said wire gauze exhibits maximumselectivity, said wire gauze comprising a rhodium/platinum alloycontaining 10% rhodium, and which when used to catalyse the oxidation ofthe ammonia in a flow of a mixture of ammonia in air, said mixturehaving an ammonia to air ratio in the range 0.56 to 0.59, at atemperature of 900° C. and a contact time of 300 ms, has a selectivityof at least 0.99.
 4. A catalyst as claimed in claim 1 wherein at leastsome of the lithium in said catalyst is in the form of a spinel, andsaid spinel is of the form LiCoO₂.